Variable attenuator



June 6, 1950 E. WEBER ET Al.

VARIABLE ATTENUATOR Awww BY e9 A .Johnson Patented June 6,A 1950 i Nj L 2,510,614

VARIABLE ATTENUATOR Ernst Weber, Mount Vernon, and Stanley A.

Johnson, New York, N. Y., assignors to Polytechnic Institute of Brooklyn, Brooklyn, N. Y.,

a corporation of New York Application JuneZl, 1945, Serial No. 600,726

This invention relates to variable attenuators for high frequency currents and especially for currents having frequencies in the ultra-high frequency range and higher.

For many applications in laboratory measurements as well as in test equipment in the iieiu, it is necessary to have variable attenuators available in order to measure power output of tubes as oscillators, to regulate the power flow into special equipment, and for other purposes.

An object of the invention is to devise a variable attenuator for high frequency currents and having a broad band characteristic.

A further object ofthe invention is to devise a variable attenuator in which good matching is obtained in all positions of adjustment and in which there is `a very low minimum value of attenuation or low insertion loss.

Still another object of the invention is to devise a variable attenuator in which there is symmetry with respect to matching at all positions of adjustment. In other Words, for all adjustments of the attenuator, the matching is substantially the same for either direction of transmission through the attenuator.

Broadly the invention involves a transmission conductor having an attenuator section of relatively high resistance providing the necessary maximum attenuation. The high resistance section is formed with a wall thickness preferably less than the depth of penetration (skin depth). A shunting sleeve surrounds the conductor and is slideable along the conductor to effectively shunt out a variable portion of the attenuator section. For the purpose of effecting matching of the attenuator for all positions of adjustment of the sleeve, the attenuator section of the conductor is provided with a matching section at one end thereof, and the shunting sleeve is provided with a similar matching section arranged so that the effective portion of the attenuator lies between said matching sections in all positions of adjustment of the shunting sleeve.

Different embodiments of the invention are illustrated in the accompanying drawing in which: y

Figure 1 is a plan view showing one construction of a variable attenuator formed as a unitary assembly adapted to be inserted in a coaxial line;

Figure 2 is a longitudinal section'al view of the arrangement shown in Figure 1 along the line 2-2;

21 Claims. (Cl. 178-44) Figure 3 is a longitudinal sectional view showing a modified form of shunting sleeve which may be used in the arrangementY of Figure 2; and

Figure 4 is a longitudinal sectional view of a modified arrangement where the variable shunting element is operated by a member extending out of a mounting stub positioned at a rightangle bend in the cable.

Referring to Figures 1 and 2, the attenuator is embodied in a runitary assembly which may be inserted in a coaxial line.Y 'I'he attenuator runit is mounted Within a cylindrical casing formed of two tubular sections la and Ib joined together by a suitable coupling Ic. The attenuator unit 2 is mounted at the center of. the casing and forms alinear section of the center conductor of a coaxial cable. This unit is formed of an insulating tube or rod having bullet-type connectors 2a. and 2b at the ends thereof which engage sockets formed in the ends of center conductor connectors 3a and 3b mounted in the two sections of the attenuator casing. Connectors 3a and 3b are supported within the casing by means of quarter-wave-stubs 3a and 3b'. I

A thin metallic film is deposited on the outer surface of the insulating tube to provide the required attenuation in the current transmitted through the attenuator assembly. The attenuator film is very thin, preferably only a fraction of the depth of penetration (skin depth), although greater thicknesses may be used up to twice the depth of penetration. As shown in Figure 2,- the film on the insulating tube of the attenuator unit 2 is divided into various sections marked by the points a, b, c, d, and e. The active attenuatorlm of small thickness is located in the section between the points c and d. The film in the section b-c is thicker than the section c-d and comprises a transformer section or matching section to prevent or cancel wave reflections produced by the attenuator section c-d. The nlm section af-b is relatively thick and is equivalent to a solid metal conductor. Likewise, the film section d-e is relatively thick and is equivalent to a solid conductor.

A shunting sleeve is mounted to slide on the attenuator unit 2, and in the arrangement shown in Figure 2, this sleeve is formed of a metallic section 4 having an insulating section 5 joined thereto. The metallic sleeve 4 is formed of very thin wall thickness consistent with mechanical requirements in order to take up as little space as possible. It is also desirable that the insulating sleeve section be as thin as possible. Between the points 5a and 5b on the insulating section 5, a thin metallic film is formed on the inner Wall of the insulating tube, and this nlm serves as a matching section equivalent to the matching section b-c on the attenuator unit 2. Generally speaking, the matching section 5a-5b on the shunting sleeve will be of the same length as the matching section b-c oni the attenuator unit. A thicker i'llm` equivalent to a solid conductor is placed on the inner wall of the insulating section 5. betweeny the point 5b' and the end of the metallic section 4' indicated at 4b, and this thick film has electrical connection with the metallic section 4 so that the shunting sleeve between the points 4a andi 5h is equivalent to a solid metallic sleeve. If desired, the metallic sleeve 4 may be made' of the" proper length to form the entire section 4a-5b. ThisAk section Should' bef suilciently long to-bridge the-portion ofV the attenuator unit 2-'bet'ween tl'ie points b and'l d; in other Words, the section la-5b of the shunting sleeve should have a length equal t'o'or greater than the combined length ofy the sections'- bc `and c-dl of the attenuator'unit 2". As shown in Figure 2, the section 4a 5b ofy the shunting sleeve is' slightly longer than the combined'` sectionsv b--dr of the a'ttenuator unitv and the sleeve isf in a position tobridge the active film* section and the` trans'- former section of the attenuator unit This is the position of the shunting sleeve for lowest attenuation'.

The distance Abetween the end of the-metallic sleeve 4fand thev end" of' thev mountingj 3U' should belong enough topermit the shunting sleeve to be m'ove'df to the right until the` point 5B on the sleeve comes opposite' thepoint d' on the' attenuator unit 2; This wouldY be the positionv of'the shunting sleeve for producing maximum attenuation With substantially equal nfiatcliing in both directions of transmission. In this position of the sleeve, the end: of the transformer'section on theshunting sleeve would be at the point' 5a", and the active attenuator film section Would then be locatedY between' the point c' and the point 5a'. As the shunting sleeve ismoved from this' position towards the left, more and more' of' the active attenuator film is shunted out by the shunting sleeve` and the attenuation is reduced accordingly'. When the shuntin'g sleevek reaches the position shown in Figure 2, the entirev active sectionV of the' attenuator film, as Well as the transformersection b-c, is bridgedl by the solid shunting portion ofthe shunting sleeve, andthe attenuation is at a minimum. With this posin tion of the'sl'eeve', the transformer section 5ft-5h of the s'hunting'sleeve surrounds the section'-b of the attenuator unit and has no substantial effect.

The present invention isnot concerned With a method of forming the diierent metallic lm's embodied in the' attenuator unit andin the shuntw ing sleeve, but We prefer to" form these lmg of an alloy: of at least tvvomoblev metals, the' alloy having substantially zero temperature coefficient. For the purpose of preventing mechanical injury to" the' various films by the sliding of theshunt ing' sleeve over theA attenuator unit, the lms shouldbe'coated with a suitable varnish' to-pre-y vent direct contact oetvve'e'nV relatively movable metallic surfaces.

For the purposeof adjusting thel position of the shunting sleeve of the attenuator unit, a handle S of insulating material is secured to the sleeve 4 and extends radially out of the casing through a longitudinal slot formed therein. A rack 'i is mounted outside of a casing la and one end thereof is connected with the handle 6 by means'o'f a suitable connecting element la. The rack may' be mounted in variousw'ays fior sliding movement parallel with the axis of the attenuator unit 2, and one suitable arrangement is illustratedin. Figures 1 and 2 Where the rackl l passes through an aperture formed in a block 8 which isrigidly mounted on the casing section la. A pinion 9 is mounted within a cavity formed in I theY block 8 and'n engages the rack '1, the pinion beingV mounted on a shaft 9a which is journaled inthe block 8'y and is provided with an operating knob IB. Thus, by turning the knob IE), the rack may be driven in either direction to adjust the position of the shunting sleeve on the attenuatorunil 2.;

A suitable:n pointer H' maybe attached to the rack 'l' and arranged tovv cooperate with azgraduated scale l2 formed onL the surface of casing section liav alongztire path ofrtravel-of: the pointer The rack l" andthe handleV 6 may bef protected from mechanical.' injury and interference by arrangingV guard: strips la'and' I--S'b'y on each side of the block` vand" over the" path oftravel: of. the raclr IA and the'liandle- Si The outer ends of these guardi strips are supported' ley-suitable blocks Hic and i3d.

Ani adjustable" stop may. be providedi to' limit the travelof therack? 'I andma'y be formed of a screw his having' threadedy engagement With` the block I-3cfandf arrangedV in line with-the end of the rack l. A' lockinut' Ma" is' provided: to4 hold the screwy int any position ofadjustment.

Fig-ure 3fshows a modified` form or the shunt ing sleeve in which: theentire sleeve is formed-of a' single insulating-sleeve 5 having a metal-nlm covering the entire in-nerlsurtace' throughout its length. The lmfbetween the points-4a and'b-is made relatively thickandf isequivalent-toa solid metal sleeve, While the lm covering the transformer section- 5ft-5bv is a thin iilmhav-ingv the necessary resistancefto-secure the desiredy matching betvveentl'ie"solidilml section and the active attenuator film section on the attenuator unit 2`. The shunting sleeve of. Figure 3 functions in the same manner as the sleeve of. Figure 2.

In-Figure 4 We have shown a modified arrangement for mountingY the attenuator unit and for varying the position oi?.` the shunting sleeve. In this arrangement, I is the outer tubular conductor of`a coaxial cable, and l is a section o'f the cable after making a right-angle turn. The attenuator unit 2' is mountedconcentrically Within the section l' of' the cable, the right end of the unit being supported" in anysuitabie manner, such as by the use of a bullet' type of connector as shown in Figure 2 and: is connected with the center conductor of the cable. The left end of the attenuator unit extends' throughl a bore formedr in a tubular support 3c' Whicl'r is supported in theend ofl cable section I by a suit-- able plug Id. The outgoing center conductor3c'y is connectedto the sleeve 3c`at a point substantially one-quarter Wave length fromthe'4 inner aceofv plug Id.

The shunting sleeve surrounding the attenuator unitf 2" isforrnedsubstantially like the shunting sleeve ofA Figure 2v and cons-istsI of aj metal sleeve 4 which passes through the bore of sleeve 3c and extends to the outside of the cable where it may be manually adjusted in position, and an insulating sleeve 5 is attached to the inner end of the metal sleeve 4 and carries the transformer section of the shunting sleeve. This transformer section is formed in the same manner as in Figure 2 and comprises a thin metallic nlm formed on the inner surface of the sleeve 5. It will be understood that the attenuator unit 2 is provided with a matching section near its right-hand end corresponding to the matching section b-c in Figure 2. If desired, a suitable rack and pinion arrangement may .be provided for adjusting the sleeve 4 to various positions on the attenuator unit 2. v

l From the description of the operation of Figure2, the operation of Figure 4 will be obvious, since movement of thek shunting sleeve will be eiective in shunting out a variable portion of the effective section of the attenuator unit 2.

We claim:

1. A variable attenuator for a coaxial cable comprising an inner transmission conductor of said cable having an attenuator section of relatively high resistance interposed between two relatively fixed low-resistance sections of said conductor and providing the necessary maximum attenuation, and a conductive sleeve surrounding said conductor and being slideable along said conductor to effectively shunt a variable portion of said attenuator section.

2. A variable attenuator according to claim 1 wherein said attenuator section is formed as a tubular conductor having a wall thickness less than twice the depth of penetration.

3. A variable attenuator comprising a tubular conductor having an attenuator section of high resistance interposed :between two relatively xed terminal sections of low resistance, and a low resistance sleeve surrounding said conductor and being slideable along said conductor to effectively shunt a variable portion of said attenuator section.

4. A variable attenuator structure comprising a tubular casing, a conductor having a low resistance tubular section joined to a high resistance tubular section of the same diameter, a conductive sleeve surrounding said low resistance tubular section and being movable along said conductor to surround said high resistance tubular section, and means for effecting movement of said sleeve from outside of said casing.

5. A variable attenuator comprising a tubular conductor having an attenuator section of high resistance interposed between two relatively xed terminal sections of low resistance, a low resistance sleeve surrounding one of said terminal sections, a matching section interposed between the other terminal section and said attenuator section, and a tubular matching section carried on the end of said sleeve nearest said first-mentioned matching section, said sleeve and the attached matching section being movable along said conductor to surround a variable length of said attenuator section.

6. A variable attenuator comprising an insulating carrier, a thin metallic lm formed on an intermediate section of said carrier, relatively thick metallic lms formed on the terminal portions of said carrier and connected with said thin nlm, a conductive sleeve surrounding said carrier and being movable along said carrier to effectively shunt a variable portion of' said thin film.

'1. A variable attenuator according to claim 6 and including a matching section of lm interposed between one terminal portion and said thin nlm on said carrier, and a matching section located at one end of said conductive sleeve.

8. A variable attenuator structure comprising, in combination, a tubular casing, a center conductor mounted within said casing and embodying an attenuator section of relatively high resistance, a sleeve of conductive material surrounding said center conductor and being movable into position surrounding said attenuator section, and means extending outside of said casing for effecting movement of said sleeve along said conductor.

9. A variable attenuator structure comprising, in combination, an outer tubular casing having a slot formed therein parallel with its axis, a center conductor mounted within said casing and embodying an attenuator section, a conductive sleeve surrounding said inner conductor and being movable into position to surround said attenuator section, and operating means secured to said sleeve and extending through said slot to the outside of said casing.

10. A variable attenuator according to claim 9 and including a rack mounted on said casing for sliding movement parallel with the axis thereof, and a pinion for operating said rack, and a pointer carried by said rack and cooperating with an indicating scale carried by said casing.

11. A variable attenuator structure comprising, in combination, an outer casing having two sections arranged at right angles to each other, a center conductor arranged within said casing, a quarter wave support for supporting said inner conductor at the bend in saidcasing, said support having a bore formed in alignment with one section of said inner conductor, an attenuator unit embodied in said one section of inner conductor, and a conducting sleeve positioned within said :bore and surrounding said one section of inner conductor, said sleeve being mova-ble in said bore to effectively shunt a variable portion of said attenuator unit.

12. An attenuator comprising a concentric transmission line and consisting of a tubular outer conductor and a centrally disposed inner conductor, said inner conductor consisting of two separated sections of low resistance, a member bridging the gap between the separated ends of said sections and serving to attenuate waves transmitted between said low resistance sections, and a conductive sleeve surrounding said inner conductor and being adjustable along said inner conductor to surround an adjustable portion of said attenuating member.

13. An attenuator according to claim 12 wherein said attenuating member is formed of a dielectric carrier provided with a high resistance coating on the surface thereof.

14. An attenuator comprising a concentric transmission line and consisting of a tubular outer conductor and a centrally disposed inner conductor, said inner conductor consisting of two separated sections of low resistance and a conductive sleeve closely surrounding one section of said inner conductor and being adjustable in position along said inner conductor to vary the gap between one end of said sleeve and the end of the other low resistance section of the inner conductor.

15. An attenuator according to claim 14 and including a rod of dielectric material interposed between the separated sections of inner conductor and having substantially the same outside diameter as said inner conductor.

1-6. An attenuator according to claim 15 wherein saiddielectric rod is provided with a high resistance coating on the surface thereof.

17. A loss-producing variable attenuator for high frequency currents comprising a high resistance conductor section having a wall thickness less than twice the depth of penetration of said currents, and a conductive shunting member arranged in closely spaced relation to said conductor section but out of contact therewith and being movable parallel therewith to provide a transmission path in4 parallel with a variable portion of said conductor section.

18. An attenuator structure for a coaxial cable having a tubular outer. conductor and a center conductor, the combination of an effective quarter-wavestub mounting for supporting said center conductor,v a. variable attenuator unit arranged within said outer conductor and having a movable element for varying. the attenuation thereof, and means located within the center support of` said stub mounting and extending out of the end of said mounting for adjusting the position of said movable element.

19. In a coaxial cable construction, the combination of a tubular outer conductor and a center conductor, an effective quarter-Wave stub mounting for supporting said center conductor Within said outer conductor, a movable element enclosed within said tubular conductor for varying the transmission characteristic of said cable, and means located within the center support of said stub mounting and extending out of the end of said mounting for adjusting the position of said, movable element.

20. A variable attenuator for high frequency currents comprising a lowl resistance transmission conductor having a high resistance section interposed between two low resistance sections, a matching section of lower resistance than said high resistance section interposed betweenA said high resistance section and one of said low resistance sections, the wall thickness of said high resistance section being less than twice theV depth of penetration, and a conductive shunting member arranged in closely spaced relation to the other low resistance conductor section but out lof contact therewith and being movable into closely spaced relation with said high resistance section to provide a transmission path in parallel with a variable portion of said high resistance section.

21. A variable attenuator according to claim 20 wherein said shunting member is provided with means at the end thereof for preventing Wave refiection.

ERNST WEBER. STANLEY A. JOHNSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,151,157 Schelkunoi Mar. 21', 1939 2,197,122 Bowen Apr. 16, 1940 2,197,123 King Apr. 1.6, 1940 2,423,461 Meahl July 8, 1947 2,427,643 Collard Sept. 16, 1947 

